Photomasks are used in a wide variety of applications including the fabrication of semiconductor integrated circuits such as ICs, LSIs and VLSIs. Basically, the photomask is prepared from a photomask blank having a chromium based light-shielding film on a transparent substrate, by forming a predetermined pattern in the light-shielding film by photolithography using UV or electron beams. The current demand for a higher level of integration in the semiconductor integrated circuit market has created a need for a finer pattern rule. The traditional solution is by reducing the wavelength of exposure light.
However, reducing the wavelength of exposure light improves resolution at the sacrifice of focal depth. This lowers the process stability and adversely affects the manufacture yield of products. One effective pattern transfer method for solving the problem is a phase shift method. A phase shift mask is used as a mask for transferring a micro-pattern.
Referring to FIGS. 8A and 8B, a phase shift mask is illustrated as comprising a substrate 1 and a phase shifter film 2 deposited thereon. The mask consists of a phase shifter 2a that forms a pattern on the substrate and an uncovered area 1a of the substrate 1 that is exposed where the phase shifter 2a is absent. A phase difference of about 180° is set between light transmitted by the uncovered substrate area 1a and light transmitted by the phase shifter 2a. Due to light interference at the pattern boundary, the light intensity at the interfering boundary becomes zero, improving the contrast of a transferred image. The phase shift method permits to increase the focal depth for acquiring the desired resolution. This achieves improvements in resolution and exposure process margin, as compared with conventional masks having ordinary light-shielding patterns in the form of chromium film.
Depending on the light transmission of phase shifter, the phase shift masks are generally divided for practical application into full transmission type phase shift masks and halftone type phase shift masks. The full transmission type phase shift masks are transparent to the exposure light wavelength because the light transmittance of the phase shifter section is equal to the light transmittance of uncovered substrate areas. In the halftone type phase shift masks, the light transmittance of the phase shifter section is several percents to several tens of percents of the light transmittance of uncovered substrate areas.
Referring to the drawings, there is illustrated the basic structure of a halftone type phase shift mask blank and a halftone type phase shift mask. The halftone type phase shift mask blank shown in FIG. 1 has a halftone phase shift film 2 formed over substantially the entire surface of a substrate 1. Patterning the phase shift film 2 results in the halftone type phase shift mask which is shown in FIG. 6 as comprising phase shifter sections 2a forming the pattern on the substrate 1 and uncovered areas 1a of the substrate where the phase shifter is absent. Light that passes the phase shifter section 2a is phase shifted relative to light that passes the uncovered substrate area 1a. The transmittance of the phase shifter section 2a is set to a light intensity that is insensitive to the resist on a wafer or article subject to pattern transfer. Accordingly, the phase shifter section 2a has a light-shielding function of substantially shielding exposure light.
The halftone phase shift masks include single-layer halftone phase shift masks featuring a simple structure and ease of manufacture. Some single-layer halftone phase shift masks known in the art have a phase shifter of MoSi base materials such as MoSiO and MoSiON as described in JP-A 7-140635.
In general, when a pattern is transferred to a resist formed on a wafer, a phase shift mask is repeatedly irradiated with exposure light within a stepper/scanner. During the process, the halftone phase shift film of the phase shift mask can be damaged by the energy of irradiating light. Then repeated irradiation of exposure light gradually leads to some deviations from the initially set phase difference and transmittance. These deviations affect the position and dimensional accuracy of a pattern to be formed on a wafer and eventually, have detrimental effects such as lower yields of device manufacture.
The exposure light irradiated through the phase shift mask has a wavelength which is selected in accordance with the fineness of a pattern to be formed on a wafer. Specifically, exposure light of a shorter wavelength, for example, in the order of 365 nm for i-line, 248 nm for KrF laser, 193 nm for ArF laser, and 157 nm for F2 laser is used as the pattern to be formed on a wafer becomes finer. The wavelength used in pattern exposure becomes shorter, which means that light with a higher level of energy is used as the exposure light. Then, the problem that the halftone phase shift film is damaged by repeated exposure becomes significant.